Cumulative Layout Shift (CLS)
CLS measures how much page content moves around unexpectedly while loading — the effect of tapping a button just as an advert loads above it and pushes it away. Google considers 0.1 or less good.
The causes are consistently the same: images and embeds without width and height set, adverts injected into unreserved space, and web fonts that swap and change text metrics. Each is fixed by reserving the space before the content arrives.
It's the vital most tied to visible frustration. Users may not articulate a slow LCP, but everyone has clicked the wrong thing because the page jumped.
Why it matters
Cheap to fix, and it directly costs conversions on mobile where mis-taps send people to the wrong page.
Related terms
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